The invention relates to the general field of turbine engines. In particular, the invention relates to controlling the fuel flow rate of a turbine engine. It applies in preferred but non-limiting manner to the turbine engines used in the field of aviation.
In known manner, the fuel flow rate injected into the combustion chamber of a turbine engine is determined by a metering device having a slide, also known as a fuel metering valve (FMV). The fuel flow rate depends on the position of the slide.
A setpoint for the weight flow rate of fuel that is to be injected into the combustion chamber is evaluated by the engine computer. A technique that is conventionally used for controlling the position of the slide on the basis of the weight flow rate setpoint delivered by the computer is as follows.
As it moves, the slide varies a fuel flow section S through the metering device. The section S is also referred to as the open area of the metering device and it is easily expressed as a function of the position of the slide. In known manner, this section S is proportional to the volume flow rate of fuel when the pressure difference across the metering device is kept constant. The volume flow rate is equal to the weight flow rate divided by the density of the fuel. The density of the fuel is generally assumed to be constant throughout a mission of the turbine engine, and to be determined.
It is therefore possible to convert the fuel weight flow rate setpoint into a slide position setpoint.
A regulation loop then compares the slide position setpoint with the real position of the slide as measured by a sensor and determines a slide control signal.
The above-outlined control technique presents several drawbacks. Firstly, it does not take account of possible variation in the nature of the fuel used, nor of possible variation in the density of the fuel, e.g. as a result of a temperature variation. Furthermore, the metering device is generally not very accurate.
As a result, the fuel weight flow rate actually injected can differ from the weight flow rate setpoint. In other words, control may be inaccurate.
It is also known to use a flow meter in the fuel circuit in order to improve control accuracy.
For example, document U.S. Pat. No. 5,305,597 proposes using a measurement delivered by a flow meter to evaluate a calibration signal proportional to the instantaneous density of the fuel.
That document also proposes using an accuracy criterion for verifying the validity of the measurement delivered by the flow meter. More precisely, the measurement delivered by the flow meter is considered as being valid while it is constant and greater than a predetermined threshold for a predetermined period. When the measurement is not considered as being valid, the most recent value to be considered valid is used for determining the calibration signal.
Nevertheless, a flow meter may be affected by various types of failure. The accuracy criterion used by the above-mentioned document does not make it possible to detect a failure of any type or to adapt the fuel flow rate command as a function of a detected failure. Thus, it does not provide satisfactory accuracy in the event of the flow meter failing.
There thus exists a need to improve the accuracy with which the fuel flow rate of a turbine engine is controlled.